We paid a 400 Eur "franquicia" to cover possible damages.  At the return, the company representative found a very minor scratch on the front bumber (I'm quite sure it was there from the beginning, but too small to notice).  Then it took them more than 4 months to pay back a part of the 400 Euro - they deducted almost 200 Euro to cover the "damage" and only paid back 4 months later after having written them several times.  Maybe they just hope the customers will have forgotten what they paid?

I also agree with the comment of others:  It is not reasonable to make clients pay for a full tank (at high price) and then just ask to "leave the tank as empty as possible" - this is probably how they make a profit to compensate for low list prices.